It is known that contact lenses, whether they are cosmetic or corrective, are widely used among the population. Among these, particular importance is given to contact lenses of the soft type, which, compared with the other types of lenses (rigid or semi-rigid) provide greater comfort in use.
This advantageous characteristic is provided by the high hydrophilic capacity of the lens which, containing a substantial percentage of water, permits greater compatibility between the lens and the eye, in addition to a greater deformability of the lens which allows it to adapt better to the surface of the eye.
Soft contact lenses are conventionally produced by means of a method which provides for a first step of obtaining a dry semi-manufactured product of lenticular shape and made of polymeric material, which may in turn be obtained by the polymerization of a monomer directly in a die (moulding technique), or by turning a disc of already polymerized material (turning technique).
Independently of the technique used for its preparation, the dry semi-manufactured product is then hydrated by immersion in a saline solution (known as a physiological solution) composed of around 1% by weight of sodium chloride in water. The polymeric material used is typically endowed with optimum hydrophilic properties, and is normally a polymeric mixture based on HEMA, so that a substantial amount of saline solution, between 25% and 75%, is absorbed in the dry semi-manufactured product.
The absorption of the liquid component, besides imparting to the lens the above-mentioned characteristics of compatibility and softness, also includes physical expansion of the dry semi-manufactured product, both radial and linear, thus determining both the final dimensions of the contact lens, and its optical properties.
The contact lens thus obtained thus comprises a solid component, defining the structural portion of the lens and composed of the polymeric material, and also a liquid component, composed of the saline solution, distributed almost uniformly in the solid component.
The marketing of the contact lens provides for the preparation of suitable packs, produced for example in the form of blister packs, in which the lenses are stored and maintained for the entire period between the end of the process of manufacturing same, and the first use thereof by a user.
The packs must therefore be arranged to maintain unaltered over time the mechanical, optical and dimensional characteristics of the lenses.
To this end, the packs typically comprise a support defining a container which is filled, at least partially, with physiological solution, in which the contact lens, in its turn, is immersed. The container is further conveniently closed by a membrane, generally constituted by a thin metallic sheet (known in the field as a “foil”), subsequently removable by the user in order to extract the lens from the container.
It is known that one of the major drawbacks arising from the use of contact lenses is represented by the sensation of ocular dryness, caused by the reduction or by the breakdown of the lachrymal film which surrounds and protects the cornea. This typically involves a sensation of discomfort and irritation which frequently forces the user to remove the lenses and which, in the long term, may also lead to changes in the lachrymal function and to other pathological disfunctions at the expense of the eye (such as, for example, corneal or conjunctival inflammation).
Also owing to this drawback, many persons are capable of wearing the contact lenses only for a very limited time or are even prevented from doing so.
In order to overcome this drawback, at least in part, various technical solutions have been developed and proposed.
A first of these solutions, for the formation of the solid component, provides for the use of polymers and/or copolymers having the highest possible hydrophilic properties. Such polymers and/or copolymers may be used for producing the entire solid component of the lens, or only as an outer coating of same. The use of such polymers, however, has a negative effect on the overall costs of production, but without succeeding in providing resolutory advantages with regard to the above-mentioned drawback. Moreover, such use may also turn out to be counter-productive in the case where the lens remains in the eye for a prolonged period, since the greater the amount of water released by the contact lens, the greater the variation in the dimensions thereof, with potential negative repercussions on its optical properties.
A second known solution is provided by the use of external solutions, known generically and overall as “lachrymal substitutes” (“artificial tears”). These latter are aqueous solutions comprising substances, generally polymers, the overall properties of which tend to reproduce, as far as possible, the properties of the natural lachrymal liquid.
In order that an aqueous solution can be defined as a lachrymal substitute, it must possess particular chemico-physical and rheological characteristics and bio-compatibility. In particular, specific and high visco-elastic and hydrophilic properties are required, as well as muco-adhesive properties (in order to remain attached for as long as possible to the mucinic component of the lachrymal film), mucomimetic properties (for simulating in the best possible manner the behaviour of the lachrymal film) and the ability to bathe the cornea, in addition, obviously, to general compatibility with the tissues of the eye.
The use of lachrymal substitutes provides for their addition as drops directly on to the eye to which the contact lens is fitted, so that the need to have recourse to this expedient is a not very pleasing eventuality to many people, without considering the further inconvenience of having to carry phials or containers with one. Moreover, the beneficial effect of the lachrymal substitutes is very limited over time, so that it is often necessary to have recourse to them several times a day and in all kinds of places.
In certain cases, provision is also made for the application of the lachrymal substitute to take place directly on the lens, both when worn and prior to its being fitted, but this expedient does not substantially modify the picture described above. In fact, the absorption of the lachrymal substitute into the lens is limited to its outermost surface layers, and is therefore retained in very small amounts and thus with a beneficial effect necessarily limited over time.
The wider view of the technical field of reference of the invention is completed by contact lenses used as vehicles for the administration through the eye of drugs for therapeutic use. Such lenses, known also as “ophthalmic dressings”, are typically contact lenses obtained in a quite conventional manner, to which are then added one or more drops of the drug to be administered. The effect of release of the drug has, however, a very limited duration. Some solutions for contact lenses are also known for use as ophthalmic dressings, in which an endeavour has been made to increase the duration of the release of the drug. One of these solutions provides for the interposition of a support soaked in the substance to be administered between a pair of contact lenses.